The invention relates in general to radio frequency power amplifiers, and more particularly to apparatus and methods for enhancing the efficiency of a radio frequency power amplifiers for use in a mobile communication device capable of operating in several modes of communication.
Portable wireless communication systems are in place in all metropolitan areas, and to a large extent areas outside of metropolitan areas. Communications service is available to members of the public, which use mobile communication devices to facilitate communication. Most commonly mobile telephone service is offered, and is generically known as cellular telephone service. Others types of communication service are being introduced, but mobile telephony is by far the most popular type of service.
There are a number of types of mobile telephony which are in use, such as analog cellular, more commonly known as AMPS, digital cellular in accordance with IS-136, and systems in accordance with the Global Specification for Mobile communications (GSM), as well as a number of others. When a user travels out of their home region, they may register for service over a non-home system in what is commonly referred to as a roaming mode. However, this is normally only possible if the type of communication service the user""s mobile communication device was designed for is offered in the non-home region. For example, if the user""s mobile communication device is designed for digital cellular, and the user is in an area that only supports analog cellular, the user will not be able to communicate using the mobile communication device.
In response to the market demand to be able to communicate over a wide geographic area, including in different countries, the industry has begun offering so called multimode mobile communication devices. A multimode mobile communication device is one that is capable of supporting more than one type of communication service, meaning it can establish communication over more than one type of air interface. The most popular multimode mobile communication devices are dual mode analog and digital cellular devices, such as Advanced Mobile Phone Service (AMPS) and North American Digital Cellular (NADC) standard IS-136 phones, and AMPS and Code Division Multiple Access (CDMA) phones such as those in accordance the NADC standard IS-95.
A problem arises in these multimode mobile communication devices because of the limitation on the available space for circuits and components. Each different mode of communication uses a different air interface, including different modulation schemes. The different modulation schemes may be as different as frequency modulation for one air interface and linear modulation for another type of air interface. Each different modulation scheme would ordinarily require the power amplifier to be configured differently to achieve optimum efficiency to increase battery life. Typically there is not enough room in a small, hand-held mobile communication device to accommodate more than one power amplifier, so in a multimode mobile communication device efficiency in one or more modes of communication is sacrificed for availability. Typically the power amplifier is designed to be efficient in what is thought to be the mode of communication that will most likely be used a majority of the time. In other operating modes battery life suffers as the result of less than optimum efficiency at the power amplifier.
Therefore a need exists for an apparatus and method of efficiently amplifying a modulated signal in a multimode mobile communication device. To optimize efficiency the amplifier characteristics must change depending on the choice of communication mode. Furthermore, the amplifying apparatus must be kept small so that it does not substantially affect the size and volume goals of present day mobile communication devices.